The present invention refers to a seat back unit for a chair, in particular a wheelchair or a stand-up wheelchair.
Chairs on which the seat back can be tilted into lying position are known already for a long time. Many of these chairs have the disadvantage that on tilting of the seat back a relative motion takes place between the back of the user and the seat back causing a so called shirt-pulling effect. In order to avoid this the U.S. Pat. No. 3,630,569 proposes a mechanism which, when the seat back is tilted rearward, moves the seat back also somewhat in direction toward the tilting axis. The chair has an arm rest which is rigidly connected to the seat back. This has the disadvantage that in lying position of the user the arm rest is extending vertically upward.
There exist wheel chairs and stand-up wheel chairs having means to prevent undesirable relative motions between the user and the surfaces of the chair. The stand-up wheelchair according to U.S. Pat. No. 5,366,036 comprises a seat back frame capable of being tilted around a tilting axis. Arm rests are pivotally connected to the seat back frame. Means are provided which on standing up or on reclining the seat back to the rear move the seat back down in direction of the tilting axis, so that no shirt-pulling effect occurs. A disadvantage of this stand-up wheelchair is that in lying position the arm rests extend vertically upward, so that many invalid users will not anymore be in position to operate the controls and will require help.
The European Patent No. EP 0 960 614 discloses a chair having a seat back provided on both sides with a bracket pivotally connected to a plate of the seat frame. The arm rests are pivotally connected to the seat back by means of a U-formed support. A stud is provided at the bottom the U-formed support. This stud engages a curved slot in the plate of the seat frame. By means of a linear motor being coupled by a linkage to both brackets of the seat back the seat back can be tilted. When the seat back is tilted also the arm rests will be tilted. The curved slot controlling the tilting of the arm rests is such arranged that e.g. on an inclination of the seat back by 60 degrees the arm rests will only be tilted by 30 degrees (FIG. 5). This chair has the disadvantage that the arm rests are not kept in practically horizontal position when the seat back is inclined. Of further disadvantage is that the chair does not permit a lateral transfer movement of the user from the chair into the bed, or vice versa, because the U-formed support with the arm rest forms an obstruction. It is also of disadvantage that on an inclination of the seat back no movement of the seat back with respect to the tilting axis takes place. Therefore, to avoid a shirt-pulling effect, the pivotal connection between the seat back and the seat frame must be located near the hip joint of the user of the chair to prevent the shirt-pulling effect. Also this causes an obstruction.